With increasing power consumption of electronic devices there is a requirement for greater efficiency over the entire load range of power converters. While higher capacity power converters are desired, improving the light load efficiency is important because the light-load condition may constitute a substantial portion of the of the total converter usage time. However, the pursuit of higher power capacity is usually the priority in power supply design and consequently light load efficiency is sacrificed for higher power performance.
The LLC resonant converter provides the highest efficiency for front-end DC/DC conversion, and it is becoming the predominant topology in many applications. To increase the power capacity of an LLC resonant converter, or to mitigate the current stress of the output capacitor, interleaving is used to parallel two or more LLC power stages [1-5]. It is expected that each LLC power stage is optimized for lower power, and a high overall output power is achieved by the total output of several LLC power stages.
Load sharing is the key problem for interleaved LLC topologies. This is because the LLC converter is frequency-controlled, and when interleaved, all the LLC stages must operate at the same switching frequency for current ripple cancellation. However, at the same switching frequency, individual LLC stages may have different output powers due to component tolerances of the resonant tank circuits of each stage. This results in current imbalance between stages, such that the interleaved LLC converter does not operate properly.
Previous work on multiphase LLCs provided several load-sharing solutions but all have limitations. For example, the load sharing method in [1] solves the load sharing problem by tracking the switching frequency point at which current balance is achieved between the two non-identical LLC stages. However, since the switching frequency becomes the control variable of the load sharing loop, the freedom for voltage regulation is lost. Therefore, an additional Buck stage is needed to control the output voltage, which degrades the efficiency. Also, this method is unsuitable for more than two paralleled LLC stages, because three or more non-identical LLC stages are unlikely to reach the same output gain at the same switching frequency.
The series-input structure in [2] automatically achieves load sharing because the phase with higher output current causes the input voltage to drop, and in turn reduces the output current to the balanced point. However in this structure, the input voltage is divided by the two phases, so for half the input voltage, the primary current will double, split by two phases; and in each phase, the primary current remains approximately the same as the single-phase LLC. Therefore, the load capacity is still limited by the resonant tank design trade-offs. Further, phase shedding in this configuration is difficult because when one phase shuts down, the input voltage of the other phase will double, exceeding the design limit.
The topology in [3] automatically achieves load sharing because the resonant tanks of all phases are tied together, and the current circulates among all the phases. However, phase shedding is difficult because if any phase is shut down, it is still connected in the network, obstructing the operation of other phases.